The invention is directed to a method for chemical-mechanical polishing of layers composed of metals from the group of platinum metals, particularly iridium. The invention is also directed to a method for planarizing and/or structuring a metal layer composed of a metal from the platinum group, particularly iridium.
In order to be able to reproducibly read out the charge stored in a storage capacitor of a memory cell, the capacitance of the storage capacitor should have a value of at least approximately 30 fF. At the same time, the lateral expanse of the capacitor must be constantly reduced for the development of DRAM memory cells in order to be able to achieve further enhancements of the storage densities. These inherently contradictory demands made on the capacitor of the memory cell lead to more and more complex structuring of the capacitor (xe2x80x9ctrench capacitorsxe2x80x9d, xe2x80x9cstack capacitorsxe2x80x9d, xe2x80x9ccrown capacitorsxe2x80x9d). Accordingly, the manufacture of the capacitor becomes more involved and, thus, more and more expensive.
Another way of assuring adequate capacitances of the storage capacitors is by employing materials having an extremely high dielectric constant between the capacitor electrodes. Recently, new materials, particularly high-xcex5 para-electrics and ferro-electrics, are therefore being employed as a dielectric instead of the traditional silicon oxide/silicon nitride, and these have a clearly higher, relative dielectric constant ( greater than 20) than the traditional silicon oxide/silicon nitride ( less than 8). Given the same capacitance, the capacitor area and, thus, the required complexity of the structuring of the capacitor can thus be clearly diminished. Significant representatives of these materials are barium strontium titanate (BST), which has a formula of (Ba, Sr)TiO3, lead zirconate titanate (PZT), which has a formula of Pb(Zr,Ti)O3 or, respectively, lanthanum-doped lead zirconate titanate or strontium bismuth tantalate (SBT), which has the formula SrBi2 Ta2O9.
In addition to traditional DRAM memory modules, ferro-electric memory arrangements, what are referred to as FRAMs, will also play an important art in the future. Compared to traditional memory arrangements such as, for example, DRAMs and SRAMs, ferro-electric memory arrangements have the advantage that the stored information is not lost but remains stored even given an interruption of the voltage supply or, respectively, the power supply. This non-volatility of the ferro-electric memory arrangements is based on the fact that the polarization impressed by an external electrical field in the ferro-electric materials is also essentially retained even after the external electrical field is shutoff. The new materials such as lead zirconate titanate (PZT), (Pb (Zr, Ti)O3) or, respectively, lanthanum-doped lead zirconate titanate or strontium bismuth tantalate (SBT), (SrBi2Ta2O9) are also utilized for the ferro-electric memory arrangements.
Unfortunately, the employment of the new para-electrics or, respectively, the ferro-electrics requires the employment of new electrode materials and barrier materials. The new para-electrics or, respectively, ferro-electrics are usually deposited on electrodes (lower electrode) that are already present. The processing ensues under high temperatures at which the materials which normally make up the capacitor electrodes, thus, for example, doped polysilicon, are easily oxidized and lose their electrically conductive properties, which would lead to the failure of the memory cell.
Due to their good oxidation resistance and/or the formation of electrically conductive oxides, 4d and 5d transition metals, particularly platinum metals such as Ru, Rh, Pd, Os, Pt and, particularly, Ir or, respectively, IrO2, are considered promising candidates that could replace the doped silicon/polysilicon as electrode material and barrier material.
Unfortunately, the aforementioned electrode and barrier materials, which are newly utilized in integrated circuits belong to a class of materials that are extremely difficult to structure. Due to their chemically inert nature, they are very difficult to etch, so that the etching erosion, even given the employment of xe2x80x9creactivexe2x80x9d gases, is based mainly or almost exclusively on the physical part of the etching. For example, iridium was previously usually structured with a dry-etching process. A significant disadvantage of this process is the lack of selectivity of the method which is caused by the high physical part of the etching. This results therein that only a slight trueness to dimension of the structures can be assured due to the erosion of the masks, which have unavoidably inclined sidewalls. Over and above this, undesired re-depositions on the substrate, on the mask or in the unit employed will occur.
Over and above this, these materials also prove extremely resistant given the employment of what are referred to as CMP processes (chemical mechanical polishing).
In typical CMP processes, a semiconductor wafer whose surface is to be polished is applied on a wafer carrier and is pressed by this onto an elastic, perforated support (polishing pad) that is attached on a polishing table. The semiconductor wafer and the polishing table and, thus, the polishing support thereby rotate in opposite direction. The polishing support contains a polishing agent (slurry) that also contains active chemical additives in addition to polishing bodies.
CMP standard methods for planarization and structuring of metal surfaces exist, for example, for tungsten and copper as well as for the materials employed as barrier layer such as Ti, TiN, Ta and TaN. The CMP processes for planarization of polysilicon, silicon oxide and silicon nitride are also in the prior art.
Such methods are disclosed, for example, in U.S. Pat. Nos. 5,527,423, 5,976,928 and 5,863,838. 5,783,489 discloses a method for eroding layers of titanium, titanium nitride and aluminum alloys. In this method, polishing agents are utilized that, in addition to containing water, contain an abrasive, for example aluminum oxide, two different oxidation agents, including, among other things, peroxide disulfate (ammonium per sulfate) and hydrogen peroxide, and a stabilizer such as, for example, various phosphone acid derivatives.
The polishing fluids employed in these methods, however, are not considered suitable for the erosion of precious metal layers due to the chemical inertness and difficult oxidizability of the precious metals and their oxides such as Pt, Ir or IrO2. Methods for chemical-mechanical polishing of layers composed of metals from the group of the platinum metals, particularly iridium, have thus not yet been disclosed.
The present invention is therefore based on the object of offering a method for chemical-mechanical polishing with which layers of metals from the group of platinum metals, particularly iridium, can be polished or, respectively, eroded and/or structured and that assures an adequately high erosion rate.
This object is achieved by a method for chemical-mechanical polishing of layers composed of metals from the platinum metals, particularly iridium with a polishing fluid. Further, a method for planarizing and/or structuring layers composed of metals from the platinum metals, particularly iridium is also disclosed. Further advantageous embodiments, developments and aspects of the present invention will be apparent from the claims, from the specification and from the attached drawings.
Inventively, a method is offered for chemical-mechanical polishing of layers composed of metals from the group of platinum metals, particularly iridium, that comprises the following steps: preparing a layer of a metal of the platinum group; preparing a polishing fluid that contains 1 through 6% by weight abrasive particles; contains 2 through 20% by weight of at least one oxidation agent that is selected from the group consisting of Ce(IV) salts, salts of chloric acid, salts of peroxodisulfuric acid and hydrogen peroxide as well as the salts of hydrogen peroxide; and contains 97 through 74% by weight water. The metal layer is chemical-mechanically polished with the assistance of the polishing fluid.
The inventive method offers a method in which the material erosion of the layer to be polished ensues by chemical and mechanical erosion and not merely by means of a chemical etching. As a result thereof, the utilization of chemical-mechanical polishing for a structuring reaction-inert and oxidation-resistant metal layers such as layers composed of metals from the platinum group, particularly iridium is enabled. With the method, for example, erosion rates of 30 through 60 nm/min can be achieved, for example, for iridium layers. The re-polishing of iridium layers having thicknesses of between 50 and 150 nm typical for memory manufacture can thus be accomplished in less than three minutes. The erosion rate of iridium in the inventive method is significantly higher than that of silicon dioxide. As a result thereof, a silicon dioxide mask can be utilized in the structuring of an iridium layer without this being significantly eroded in the CMP step and losing its dimensional precision due to the edge slope.
Over and above this the polishing fluid employed in the method exhibits only slight corosivity and thus enables the utilization of traditional materials and utensils, which are regularly utilized for chemical-mechanical polishing.
The inventive CMP method achieves surface roughnesses that are comparable to those of an already established Cu-CMP process. Thus, the roughness usually amounts to about 0.5 through 2 nm and the maximum depth of scratches is approximately 5 through 15 nm.
The abrasive particles preferably exhibit a Mohs hardness of more than 7. When the inventive method is employed for chemical-mechanical polishing of iridium, it is preferred when the abrasive particles exhibit a Mohs hardness of more than 9. The term xe2x80x9cMohs hardnessxe2x80x9d as employed in this application refers to the hardness classification of hard materials according to the Mohs""s hardness scale.
In a preferred embodiment of the present invention, the polishing fluid additionally comprises 2 through 10% by weight of a stabilizer selected from the group consisting of polyacrylic acid and derivatives of phosphonic acid. This stabilizer assures a stable suspension of the abrasive in the polishing fluid. The proportion of the stabilizer in the polishing fluid preferably amounts to between 2 and 5% by weight, and particularly 3% by weight. Polyacrylic acid having a mol weight of 500 through 10000 g/mol, preferably 1000 through 5000 g/mol, and particularly approximately 2000 g/mol, is preferably utilized as the stabilizer.
In a preferred embodiment of the invention, abrasive particles are selected from the group comprising a-aluminum oxide and/or diamond. In general, abrasive particles of one type or mixtures from the various suitable types of abrasive particles can be employed in the inventive method. The proportion of abrasive particles in the polishing solution preferably amounts to between 1 and 3% by weight, and particularly 1% by weight. The abrasive particles utilized in the inventive method preferably have a diameter of less than approximately 1 xcexcm.
In other preferred embodiments of the inventive method, the Ce(VI) salt, for example ammonium cerium nitrate, (NH4)2Ce(NO3)6, is employed as the oxidation agent. In this case, the polishing fluid preferably contains 5 through 20% by weight, and preferably 7 through 15% by weight, and particularly 10% by weight, of the oxidation agent.
The employment of potassium chlorate, KClO4, as the oxidation agent is likewise preferred. In this case, a proportion of up to 15% by weight, preferably 5 through 10% by weight, and particularly 10% by weight potassium chloride is preferred in the polishing fluid. In the embodiments of the inventive method wherein salts of peroxodisulfuric acid are employed as the oxidation agent, the employment of ammonium peroxide disulfate, (NH4)2S2O8, is preferred. A proportion of 2 through 10% by weight, preferably 5 through 10% by weight, and particularly 10% by weight of the oxidation agent in the polishing fluid is thereby preferred.
When hydrogen peroxide, H2O2, is utilized in the inventive method as the oxidation agent, then the proportion thereof in the polishing fluid preferably amounts to 2 through 10% by weight, and particularly 5% by weight.
In another advantageous development of the inventive method, the pH value of the polishing fluid is greater than 1; a pH value of between 2 and 12 is preferred; and a pH value of between 4 and 9 is especially preferred.
The chemical-mechanical polishing step in the inventive method is preferably implemented given a polishing pressure of 20.7 through 82.8 kPa (3 through 12 psi), preferably 34.5 through 69 kPa (5 through 10 psi), and particularly 41.4 through 55.2 kPa (6 through 8 psi) with an additional pressure onto the backside of the wafer (back-pressure) of 0 through 34.5 kPa (0 through 5 psi), and particularly 20.7 through 27.6 kPa (3 through 4 psi). The velocities of the wafer carriers and of the polishing plate employed in the chemical-mechanical polishing step of the inventive method preferably amounts to 20 through 70 rpm; preferably 40 through 70 rpm, and particularly 60 through 70 rpm. The flow rate of the polishing fluid onto the surface of the substrate to be polished preferably amounts to 50 through 150 ml/min, and particularly 60 through 100 ml/min, during the chemical-mechanical polishing.
Inventively, a method is also offered for planarization and/or structuring of a metal layer composed of a metal for the platinum group, particularly iridium, that comprises the following steps: preparing a pre-structured substrate; applying a metal layer composed of a metal from the platinum group, particularly iridium on the surface of the substrate, at least in sub-regions thereof and planarizing and/or structuring the metal layer composed of the metal of the platinum group according to the method for chemical-mechanical polishing of layers composed of metals from the platinum metals, particularly iridium.
This inventive method offers the advantage that electrodes and barriers for highly integrated DRAMs of platinum metals, particularly iridium, can also be structured with CMP steps while avoiding dry-etching steps. Due to the high selectivity of the CMP steps between iridium and silicon dioxide, this can be set such that it has nearly ended as soon as the mask surface of silicon dioxide has been reached. When the CMP process is ended at this point in time, then the iridium layer to be structured is obtained such as prescribed by the mask surface. Geometry distortions due to chemical or mechanically attaching of the silicon dioxide masks are largely precluded.